Piezoelectric elements have an excellent conversion ratio of electrical energy to mechanical energy, and excellent responsiveness. Various piezoelectric actuators that utilize the piezoelectric effects of piezoelectric elements have therefore been recently under development.
These piezoelectric actuators have as their primary constituent element a vibrating body having a piezoelectric element, wherein the vibrating body is composed of, for example, a plate-shaped reinforcing plate having a protrusion that is in contact with a driven body at one end, piezoelectric elements affixed to both surfaces of the reinforcing plate, drive electrodes provided on the surfaces of these piezoelectric elements, and detection electrodes that are electrically insulated from the drive electrodes. A drive apparatus for a piezoelectric actuator is also known whereby a prescribed alternating current is applied to the drive electrodes of the vibrating body, the vibrating body is excited by longitudinal vibration that causes it to expand and contract in the longitudinal direction, and flexural vibration is induced whereby the vibrating body oscillates in the direction orthogonal to the direction of the longitudinal vibration (see Prior Art 1, for example).
With the drive control performed by this type of drive apparatus, a piezoelectric actuator rotates so that the protrusion of the vibrating body traces an elliptical orbit, and drives the driven body that is in contact with the protrusion. An alternating current voltage having the optimum oscillation frequency according to design must be applied to the vibrating body of the piezoelectric actuator, and the prescribed longitudinal vibration and flexural vibration must be generated therein in order for the driven body to be driven at high efficiency. However, due to the individual differences in the piezoelectric actuators, their temperature characteristics, load characteristics, and the like, it is difficult to continually apply the optimum drive frequency for which the apparatus is designed. Feedback control is therefore implemented in this drive apparatus whereby a detection signal from the detection electrodes provided to the piezoelectric elements is detected, and the drive frequency of the alternating current voltage applied to the drive electrodes is adjusted based on the detection signal.
Specifically, drive apparatuses are known that utilize the fact that the phase difference between the phase of the alternating current voltage applied to the drive electrodes and the phase of the detection signal detected from the detection electrodes, or the phase difference among the detection signals detected from a plurality of detection electrodes, is dependent upon the drive frequency of the alternating current voltage applied to the drive electrodes.
The drive apparatus of this piezoelectric actuator includes a waveform shaping circuit, a phase difference-DC conversion circuit, a comparison circuit, an integration circuit, a variable frequency oscillating circuit, and a drive circuit, as shown in FIG. 19.
The waveforms of a drive signal SDR outputted from the drive circuit and a detection signal SD1 detected from a detection electrode of the piezoelectric actuator are shaped by the waveform shaping circuit, the phase difference in these signals is detected by the phase difference-DC conversion circuit, the detected phase difference is compared with a target phase difference by the comparison circuit, and the comparison information is integrated by the integration circuit and outputted to the variable frequency oscillating circuit.
The variable frequency oscillating circuit oscillates at a frequency corresponding to the comparison information from the integration circuit and outputs a frequency signal to the drive circuit, and a drive signal corresponding to the frequency is outputted from the drive circuit to a vibrating body.
Therefore, in the drive apparatus of the piezoelectric actuator, the drive frequency of the alternating current voltage applied to the drive electrode is adjusted so that the phase difference detected by the phase difference-DC conversion circuit nears a preset reference phase difference. As a result of implementing such feedback control, an alternating current voltage having the optimum drive frequency can be applied to the vibrating body of the piezoelectric actuator, the piezoelectric actuator can be excited at prescribed longitudinal and flexural vibrations, and the driven body can be driven with high efficiency.
[Prior Art 1] Japanese Patent Application Laid-Open No. 2002-291264 (FIG. 26)